Electronic matrix arrays are usually used in devices such as liquid crystal displays (LCD). Such devices are typically arranged in row and column address lines. These lines are horizontally and vertically spaced apart and cross at an angle to one another and a plurality of crossover points are formed. Each crossover point is associated with a corresponding display element to be selectively addressed. The display element may be a pixel of an imager array, or alternatively a pixel of an LCD. A switching or isolation device such as a thin film transistor (TFT) is associated with each display element allowing individual pixels in the LCD to be selectively addressed.
Structurally, these TFTs typically include a source electrode, a drain electrode, and a gate electrode, with a thin film of semiconductor material (e.g. amorphous silicon or a-Si) disposed between the source and drain electrodes. The gate electrode in proximity to the semiconductor but electrically insulated by a gate insulator. Current flow through the TFT between the source and drain electrodes is controlled by the application of voltage to the gate electrode. The application of a positive voltage (e.g. +10 volts) to the gate of the TFT forms a conducting channel and allows current to flow between the source and drain electrodes of the TFT.
The drain electrode of a TFT is usually in electrical communication with a pixel electrode. Thus, the source electrode of the TFT is usually in electrical communication with an image signal input. In an LCD applications for example, when a voltage (e.g. +10 volts) is applied to the gate and at the same time a video voltage (e.g. +5 volts) is applied through the image signal input to the source of a TFT, a conductive channel is formed in the semiconductor layer and current flows through from the drain electrode to the source electrode. This current charges the corresponding pixel electrode of the LCD causing the pixel to be in an “on-state.” In LCD applications, the drain typically reaches a voltage similar to that is supplied to the source through the image signal input in the on-state. The amplitude of the voltage applied to the source through the image signal input thus determines how much voltage will be applied across the liquid crystal material in a given pixel and thus controls gray scale levels of the display. When voltage is no longer applied to the gate, the pixel stops charging but remains on until the next frame.
Conventionally, the TFT is made of an “island out” structure 200 as illustrated in FIGS. 5A, 5B and 5C. This structure is directly responsible for the generation of the undesirable leak photo current that reduces the performance of the TFT LCDs. FIG. 5A shows a top view and FIG. 5B shows a sectional view of the “island out” TFT structure 200. As illustrated in FIG. 5B, a gate electrode metal portion 210 is formed on a substrate 205. A gate insulation layer 212 is formed on the gate electrode metal portion 210. Additionally, a layer of intrinsic semiconductor 214 and a layer of doped semiconductor 216 are formed on the gate insulation layer 212. Moreover, a conductive layer 218 for source electrode 202 and drain electrode 204 covers the layer of intrinsic semiconductor 214 and the layer of doped semiconductor 216. FIG. 5C shows a detailed sectional view of an “island out” TFT structure 200 along the B-B′ plane as shown in FIG. 5A. A passivation layer 220 and a transparent conductive layer (i.e. indium, tin oxide, or ITO) 222 are formed on top of the TFT structure surface shown in FIG. 5B.
As it is known to those skilled in the art, one of the characteristics of the semiconductor amorphous silicon using the “island out” TFT structure is that the amorphous silicon produces “photo current” (i.e. a leakage current from the source to the drain through the semiconductor layer when the pixel is in an on-state) under normal or strong lighting. Accordingly, a TFT's photo leakage current is a critical element determining the overall image quality of LCDs. It is well known that high TFT leakage current degrades performance of an LCD display. The adverse effects include inconsistent/non-uniform gray scales, crosstalk, shading, flicker, and/or image sticking. This leakage current is an undesirable and unintentional characteristics associated with conventional TFT made of semiconductor materials such as amorphous silicon or a-Si and using the “island out” TFT structure.
Various attempts have been made to minimize TFT leakage current. For example, for crystalline-silicon and polycrystalline silicon TFTs, lightly doped drain or drain offset structures have been experimented. However, that approach requires additional process steps (i.e. photo, ion implantation, etc.). An alternative is to use an “island in” TFT structure 300 as illustrated in FIGS. 6A and 6B. FIG. 6A shows a top view and FIG. 6B shows a sectional view of the “island in” TFT structure 300. A gate electrode metal portion 310 is formed on a substrate 305. Additionally, a gate insulation layer 312 is formed on the gate electrode metal portion 310. A layer of intrinsic semiconductor 314 and a layer of doped semiconductor 316 are formed on the gate insulation layer 312. As shown in FIG. 6B, the layer of intrinsic semiconductor 314 and the layer of doped semiconductor 316 are rather narrow in comparison with the corresponding counterparts of the “island out” TFT structure shown in FIG. 5B. A conductive layer 318 for source electrode 302 and drain electrode 304 covers the layer of intrinsic semiconductor 314 and the layer of doped semiconductor 316. Since the size of the intrinsic semiconductor (amorphous silicon) is reduced, the amorphous silicon's exposure to the light is greatly reduced. Therefore, the photo leakage current is reduced as well.
Although the “island in” TFT structure reduces the photo leakage current, it requires additional mask, photolithographic and etching processes, which makes it undesirable for mass production. In order to consolidate the processing steps in mass production, normally intrinsic amorphous silicon and the source and drain electrode metal are deposited on the gate insulation layer at the same time. This process dictates that a TFT made by this process must use the “island out” TFT structure.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies, which is to find a new TFT structure and a method for making the same with reduced leakage current in the TFT and without additional TFT manufacturing and/or processing steps.